In typical acoustic investigations of earth formations from inside a borehole a tool is used with an acoustic pulse generating transmitter and a plurality of acoustic receivers. The receivers are spaced from the transmitter and produce waveforms representative of the acoustic waves incident upon the receivers after having traveled through the earth formations and the borehole. The acoustic waves arriving at the receivers include those of different wave propagation modes such as the compressional, shear and Stoneley waves. The Stoneley waves are deemed as late arrivals, i.e. waves whose velocity is less (or whose slowness is greater) than the direct or mud wave (see U.S. Pat. No. 4,131,875 to Ingram). The acoustic tool may be constructed with a acoustic pulse transmitter and several receivers such as four in the Ingram patent or twelve as described in an article entitled "Semblance Processing Of Borehole Acoustic Array Data" by Kimball and Marzetta and published in Geophysics Vol. 49, No. 3, March 1984, and in U.S. Pat. No. 4,594,691 entitled "Sonic Well Logging" which will issue on June 10, 1986.
Late arrival waves, such as zero order, low frequency Stoneley waves are boundary waves whose amplitudes in homogeneous and non-fractured earth formations exhibit little or no attenuation over the transmitter-receiver spacings normally encountered with acoustic borehole logging tools. Such late arrival waves, however, are known to be sensitive to variations in the borehole diameter, the permeability of the earth formation and the presence of fractures. Such sensitivity is revealed by a change in the amplitude of the late arrival wave.
For example, an article entitled "Synthetic Microseismograms; Logging In Porous Formations," by J. H. Rosenbaum and published in Geophysics Vol. 39, No. 1 (February, 1974) at pages 14-32 describes a theoretical technique using Stoneley waves to obtain an indication of permeability from a measurement of their attenuation. U.S. Pat. No. 4,432,077 and an article entitled "A Long Spaced Acoustic Logging Tool" by Williams et al and presented at the SPWLA twenty-fifth annual logging symposium held in June 10-13, 1984 describe a technique to obtain an indication of permeability by measuring amplitude loss or energy loss of the tube wave and using the ratio of these parameters as detected by two vertically spaced receivers located in a borehole. A standard well known variable density display (VDL) of detected acoustic waves is shown and an increased signal-to-noise ratio is obtained by real-time stacking of eight waveforms generated by each receiver and attributable to different transmitter pulses. E. A. Koerperich presented a paper published at pages 1199-1207 in the August, 1978 issue of the Journal of Petroleum Technology entitled "Investigation of Acoustic Boundary Waves and Interference Patterns as Techniques for Detecting Fractures". In this article the use of low frequency Stoneley waves to identify fractures is suggested.
Techniques have been described for using an acoustic logging tool with a receiver array containing a large number of linearly spaced receives, such as 8 or 12. For instance, U.S. Pat. No. 4,543,648 to Hsu describes a technique in which overlapping values of coherence information contained in waveforms generated by sub-arrays of receivers in response to a series of pulses generated by the acoustic source are stacked to enhance the measurement of slowness values relevant to a particular depth interval.